1. Field of the Invention
This invention relates to the fabrication of semiconductor devices, and more specifically, to the growth of high quality oxides from the surface of a semiconductor substrate.
2. Description of Related Art
The importance of high quality oxides in the fabrication of semiconductor devices cannot be overemphasized. Many broad categories of commercial devices, such as Electrically Erasable Programmable Read-Only Memories (EEPROMS), Dynamic Random Access Memories (DRAMs), and more recently, even high-speed basic logic functions, owe their commercialization to the reproducibility of high quality, very thin oxide layers.
Major improvements in gate oxide quality have been achieved by improved cleaning techniques, the addition of HCL/TCA to the gate oxidation process, and higher purity gasses and chemicals. RCA cleaning techniques are described in "Dependence of Thin Oxide Quality on Surface Micro-Roughness" by T. Ohmi, et. al., IEEE Transactions on Electron Devices, Vol. 39, Number 3, March 1992. Other techniques have incorporated different gas (NH.sub.3, ONO, WET O.sub.2) schemes in the gate oxidation cycle other than the conventional O.sub.2 with HCL or TCA. Also considerable progress has been made with single wafer RTA gate processing, as is described in "Effect of Rapid Thermal Reoxidation on the Electrical Properties of Rapid Thermally Nitrided Thin-Gate Oxides", by A. Joshi, et. al., IEEE Transactions on Electron Devices, Vol. 39, Number 4, April 1992.
These techniques refer to "gate oxides" as in the gate of an MOS transistor, but are usually applicable to any thin (usually less than 300.ANG.) oxide. The "tunnel" oxide of an EEPROM process technology is a very thin gate oxide (usually less than 100.ANG.), with the somewhat unusual requirement that it be grown above a very heavily-doped N+ region. Oxides grown from heavily-doped substrate surfaces are generally considered to be lower in quality than those grown from more lightly doped surfaces, as would be the case for most MOS transistor processes.
The growing and subsequent removing of a KOOI oxide is used to eliminate the remnant KOOI ribbon of nitride which forms around the active area at the LOCOS edge (or field edge) during the previous field oxidation. (Silicon nitride in a steam oxidation environment decomposes into ammonia and silicon dioxide. The ammonia diffuses down through the field oxide until reaching the silicon surface, where it reacts to form a silicon nitride, and leaving a ribbon of nitride at the silicon/silicon dioxide interface around the edge of the active area.) In a non-EEPROM CMOS process (not incorporating a tunnel oxide for EEPROM structures) the KOOI oxide is commonly used as an implant oxide for V.sub.T controlling implants, and is subsequently removed and followed by the gate oxidation and polysilicon deposition for FET's.
Unfortunately, processes adequate for fabricating devices having high quality thin oxides are generally not adequate for fabricating devices having very thin high quality oxides.